The present invention relates to a process for removing nitrogen oxides (NOx), and more particularly to a process for removing NOx by selective reduction from gases having a low temperature (e.g., of up to 300xc2x0 C.) and a nitrogen dioxide (NO2)/NOx ratio in excess of 0.5, such as combustion exhaust gases produced in starting up gas turbines, regeneration exhaust gases containing removed NOx and resulting from the regeneration of NOx adsorbents by heating, and process exhaust gases in various modes of chemistry of nitric acid.
It is conventional practice to use a vanadium-tungsten supporting titania catalyst and a reducing agent, such as ammonia or urea, for reducing and decomposing NO and/NO2 in the gas to be treated for the removal of NOx.
However, this denitration process has the problem that the catalytic activity is lower when the component molar ratio of NOx in the gas to be treated is NO2 greater than NO than when the ratio is NO2xe2x89xa6NO as will be described below.
FIG. 1 shows the relationship between the NO/NOx ratio and the denitration efficiency.
Denitration conditions:
Areal velocity (AV) 35 Nm3/m2xc2x7h
Composition of the gas to be treated
Air+H2O (about 3%)
NOx 90 ppm
NH3 90 ppm
Reaction temperature 250xc2x0 C.
The graph shows that the denitration efficiency becomes maximum when the NO/NOx ratio is 0.5 (No:No2=1:1) and lowers as the NO/NOx ratio decreases from 0.5. One of the causes of the reduction in the catalytic activity is thought attributable to a diminution in NH3 adsorption sites due to an excess of oxygen on the catalyst active sites as will be described below.
1) In the case of removal of NO
NO+NH3+1/4O2xe2x86x92N2+3/2H2O
Although the catalyst active sites are reduced to result in a deficiency of oxygen, the active sites are reoxidized with the oxygen in the gas to be treated and are thereby replenished with oxygen. If the reaction temperature has a low value of up to 200xc2x0 C., difficulty is encountered in oxidizing the catalyst with this gaseous-phase oxygen to result in markedly impaired denitrating properties.
2) In the case of removal of NO2
NO2+NH3xe2x86x92N2+3/2H2O+1/4O2
When the gas to be treated contains oxygen in a high concentration, the oxygen produced on the catalyst active sites is not readily releasable into the gaseous phase. An excess of oxygen on the catalyst active sites therefore inhibits the adsorption of ammonia, consequently impairing the denitrating properties of the catalyst.
3) In the case of denitration of NO+NO2 (1:1 in molar ratio)
NO+NO2+2NH3xe2x86x922N2+3H2O
There is no excess or deficiency of oxygen, permitting the catalyst to exhibit the highest denitrating properties.
An object of the present invention is to provide a process for removing NOx which is free of impairment in denitration efficiency at a reaction temperature of up to 300xc2x0 C. even when the component molar ratio of NOx in the gas to be treated is NO2 greater than NO.
In removing NOx from the gas to be treated and containing NO2 in a larger amount than NO, that is, having a (NO2)/NOx ratio in excess of 0.5, by selective reduction with use of ammonia serving as a main reducing agent in the presence of a denitration catalyst, a process for removing NOx which is characterized by adding to the denitration reaction system a substance for removing an excess of oxygen accumulating on catalyst active sites by selectively reducing the oxygen at not higher than 300xc2x0 C., for example, at 300 to 150xc2x0 C., in other words, a substance which reacts with the excess of oxygen on the catalyst active sites and becomes oxidized at not higher than 300xc2x0 C. (the substance will be referred to as an xe2x80x9cauxiliary reducing agentxe2x80x9d).
The auxiliary reducing agent is a substance which reacts with the excess of oxygen on the catalyst active sites and becomes oxidized at not higher than 300xc2x0 C., irrespective of gaseous-phase oxygen. Preferably, the agent is an organic compound.
It is desired that the auxiliary reducing agent or a liquid containing the agent (e.g., aqueous solution, to be used in the same meaning hereinafter) be present in the form of a vapor or gas before reaching the denitration catalyst, as uniformly diffused. Accordingly, it is desired to introduce the auxiliary reducing agent into the system, for example, by:
injecting the agent or the liquid containing the agent directly into the flow of gas to be treated, or
injecting the agent or the liquid containing the agent into a stream of air for diluting ammonia as the main reducing agent and forcing the agent or liquid into the flow of gas to be treated along with the ammonia.
In the case where the auxiliary reducing agent is a liquid, the amount of injection may be controlled by feeding the agent or the liquid containing the agent to the NOx removal apparatus by a metering pump, detecting the concentration of NOx (NO, NO2) at the inlet of the apparatus, and controlling the pump with the resulting detection signal so as to alter the operating conditions such as the stroke, pitch, etc. of the pump.
When the auxiliary reducing agent or the liquid containing the agent is injected into the ammonia diluting air stream, it is likely that the agent or liquid will not be evaporated completely. It is then desirable to preheat the ammonia diluting air before the agent or liquid is injected. Instead of preheating the ammonia diluting air, it is also desirable to admix a portion of the gas of high temperature to be treated with the ammonia diluting air.
Aqueous ammonia or aqueous solution of urea is also usable as the ammonia supply source. In this case, it is desired to dissolve the auxiliary reducing agent in the aqueous solution first to add the agent and NH3 to the denitration reaction system at the same time.
The auxiliary reducing agent is a substance which is not oxidized with gaseous-phase oxygen at a low temperature (up to 300xc2x0 C.) but selectively reacts with an excess of oxygen on the catalyst active sites.
The preferred auxiliary reducing agents include hydrocarbons and alcohols.
Examples of hydrocarbons are lower alkanes having 1 to 10 carbon atoms, such as ethane, propane, butane, pentane and hexane; lower alkenes having 2 to 10 carbon atoms, such as ethylene, propylene, butene, pentene and hexene; and saturated or unsaturated hydrocarbons such as derivatives of these compounds.
Alcohols are useful insofar as they are compounds having one or at least two hydroxyl groups. Examples of these alcohols are primary alcohols, secondary alcohols or tertiary alcohols having 1 to 10 carbon atoms, such as methanol, ethanol, propanol, butanol, pentanol and hexanol; and alcohols such as derivatives of these alcohols. Useful alcohols may be monohydric alcohols, dihydric alcohols or polyhydric alcohols. Aromatic alcohols are also usable. Especially desirable are monohydric alcohols having 1 to 10 carbon atoms.
It is desired that the amount of the auxiliary reducing agent to be injected be as small as possible in view of the occurrence of unreacted substances and formation of by-products. Stated more specifically, the useful amount of injection is at least an amount capable of consuming by an oxidation reaction xc2xd mole of excessive oxygen to be produced when 1 mole of nitrogen dioxide (NO2) is removed. Further in the presence of nitrogen monoxide (NO), xc2xd mole of excessive oxygen is consumed by 1 mole of NO, so that the amount of the auxiliary reducing agent to be injected is not smaller than is capable of consuming the excessive oxygen resulting from the difference of [amount of NO2xe2x88x92amount of NO]. For example, in the case where isopropanol is used as the auxiliary reducing agent and when the component molar ratio (NO/NOx)=0 (i.e., NO2 only), the amount is preferably at least {fraction (1/9)} mole to not greater than xc2xd mole per mole of NO2. When the component molar ratio is in the range of 0 greater than (NO/NOx) less than 0.5 in this case, the amount is preferably up to {fraction (1/9)} mole per mole of NO2.
The preferred amount of the auxiliary reducing agent to be injected is not smaller than the stoichiometric amount required for consuming the excessive oxygen to be produced by the reaction between NO2 and ammonia to not greater than the amount of NOx.
When an excess of the auxiliary reducing agent is injected, the excessive oxygen is removed from the catalyst active sites rapidly to give higher denitrating properties to the catalyst, whereas if used in an amount larger than the amount of NO2 in NOx (=NO+NO2), the agent will reduce the catalyst active sites to excess and is likely impair the denitrating properties (especially the NO removing property) of the catalyst.
The denitration catalyst may be one enhanced in oxidizing ability so as to readily oxidize the auxiliary reducing agent, and is not limited specifically. Examples of preferred catalysts include a titania catalyst having vanadium supported thereon.
Ammonia undergoes an equimolar reaction with NO or NO2 and is therefore injected in an amount calculated from: (amount of NOx at inlet)xc3x97(required denitration efficiency)+(allowable amount of leak ammonia). Thus, the amount of ammonia to be injected is dependent on the amount of NOx to be removed and is not always limited to the foregoing range.
In removing NO2 from the gas to be treated by selective catalytic reduction with use of NH3serving as a main reducing agent, the equilibrium relation of NO2=NO+1/2O2 produces NO in the case where the denitration reaction temperature is high (in excess of 300xc2x0 C.). Furthermore, the combustion of ammonia in this case also produces NO through the reaction of:
NH3+5/4O2xe2x86x92NO+3/2H2O
These portions of NO produced consume the excessive oxygen on the catalyst active sites. Accordingly, the nitrating properties will not be impaired greatly even when the component molar ratio is NO2 greater than NO.
In the case where the denitration reaction temperature is low (up to 300xc2x0 C.), on the other hand, formation of NO is almost unexpectable, so that an excess of oxygen inhibiting the adsorption of ammonia is produced on the catalyst active sites in removing NO2.
According to the process of the present invention, an auxiliary reducing agent is used which is a substance to be oxidized with the excess of oxygen on the catalyst active sites at not higher than 300xc2x0 C., so that when the NOx in the gas of low temperature to be treated is in the range of NO2 greater than NO in component molar ratio, the excessive oxygen produced on the catalyst active sites is consumed for the oxidation of the auxiliary reducing agent, consequently obviating the likelihood that the excessive oxygen will inhibit the adsorption of ammonia by the active sites. This ensures ammonia adsorption on the catalysts active sites.